This invention relates generally to an apparatus for fusing images on copy substrates, and more particularly to such an apparatus, which affects fusing by the combined application of heat and pressure. This fusing apparatus is suitable for use in an electrostatographic recording machine such as, for example, a xerographic copier.
In a xerographic copier a light image of an original document to be reproduced is recorded in the form of a latent electrostatic image on a photosensitive member. The latent image is rendered visible by the application of a resin-based powder known as toner. The visual toner image is transferred electrostatically from the photosensitive member on to sheets of paper or other substrates. The toner image is then fixed or "fused", for example by applying heat and pressure, which causes the toner material to become soft and tacky whereby it is able to flow into the fibers or pores of the substrate or otherwise upon the surface thereof. Thereafter, as the toner material cools, it solidifies and is bonded firmly to the substrate. In the electrostatographic art generally the use of thermal energy and pressure for fixing toner images on to a substrate is well known.
It has long been recognized that one of the fastest and most positive methods of applying both heat and pressure for fusing the toner image to the substrate is by direct contact of the resin-based toner image with a hot surface such as a heat roller which also applies pressure to the substrate. One approach is to pass the substrate with the toner image thereon between a pair of opposed rollers forming a nip, at least one of the rollers being internally heated. The actual temperature and pressure ranges will of course vary depending upon the softening range of the particular resin used in the toner. Typically, however, it will be necessary to heat the toner powder above 180.degree. C. Temperatures of 198.degree. C. or even higher are not uncommon in commercial fusers. Corresponding nip pressures are in the range of 690 to 1380 kNm2.
A problem with this kind of fuser is that, as the toner becomes tacky, it can stick to the surface of the fuser roller which is undesirable because some of the toner on the fuser roller can then be transferred to subsequent substrates being fused and, moreover, those subsequent substrates will in their turn give rise to even more toner sticking to the fuser roller. This effect, known as "offset", clearly impairs copy quality. Furthermore, if the rollers are rotated when there is no substrate present in the nip therebetween, toner may also be transferred from the fuser roller to the backup roller so that when a substrate subsequently passes through the nip some of the toner may be transferred to the reverse side thereof.
An arrangement for minimizing the problem of offset has been to provide a fuser roller with an outer surface or covering of, for example, polytetrafluoroethylene known by the trade name Teflon, to which a liquid release agent such as silicone oil is applied. The thickness of the Teflon is typically of the order of tens of microns and the thickness of the oil is less than 1 micron. Silicone based oils, for example polydimethylsiloxane, which possess a relatively low surface energy, have been found to be suitable for use in the heated fuser roller enviroment where Teflon constitutes the outer surface of the fuser roller. In practice, a thin layer of silicone oil is applied to the surface of the heated roller to form an interface between the roller surface and the toner images carried on the substrate. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip thereby preventing toner from offsetting to the fuser roller surface.
In attempts to improve the quality of the image fused by a heat roller fuser, such rollers have been provided with conformable surfaces comprising silicone rubber or Viton (Trademark of E I Du Pont for a series of fluoroelastomers based on the copolymer of vinyladinefluoride and hexafluoropropylene). As in the case of the Teflon coated fuser roller, release fluids such as silicone based oils are applied to the surface of the silicone rubber or Viton to both minimize offsetting and to facilitate stripping. When the fuser system is one which provides for applying silicone oil to silicone rubber or Viton, a low viscosity silicone oil (i.e. in the order of 100 to 1000 centistokes) has most commonly been employed, although liquids of relatively high viscosity, for example 12,000 to 60,000 centistokes and higher, have also been used.
Various forms of applicator have been employed to supply the liquid release agent to the surface of the fuser roller. Thus, for example, U.S. Pat. No. 4 231 653 discloses an applicator comprising an elongate trough for containing a supply of release oil. A wick which is partially immersed in the release oil supply draws the oil up from the trough for application to the fuser via a pair of cooperating rollers in pressure contact, namely a driven oil application roller and a freely rotatable oil supply roller. The wick is in engagement with the oil supply roller and thus applies the release oil directly to the surface thereof. The oil supply roller slips on the application roller and is not rotated when there is some oil present between the two rollers, but as the oil runs out the oil supply roller is driven by the oil application roller since the coefficient of friction therebetween is increased. In other words, the oil supply roller is rotated only when there is little or no oil on the surface of the oil application roller due to the application of oil to the fuser and thus the cooperating roller pair acts as a metering device for checking the amount of release oil conveyed to the fuser.
U.S. Pat. No. 4 050 801 discloses a release oil applicator also comprising an elongate trough containing a supply of release oil. Again the oil is drawn up by a wick which is partially immersed therein but, in this case, the wick is in engagement with the fuser roller so that the oil is applied directly to the surface thereof. The amount of release oil is checked once it has actually been applied to the fuser roller and for this purpose there is employed a doctor blade disposed adjacent the surface of the fuser roller, which scrapes off any surplus oil. The doctor blade may be made of any suitable material, e,g, a fluorosilicate elastomer.
U.S. Pat. No. 4 214 549 discloses an applicator in which release oil is contained in a trough-like sump from which it is dispensed by means of a metering roller which cooperates with a donor roller. The metering roller is partially immersed in the oil in the sump and the donor roller contacts the surface of the heated fuser roller. A wick which is fully immersed in the oil in the trough contacts the metering roller to promote the application of oil thereto. A doctor blade fabricated from VITON (trademark) contacts the metering roller and checks the thickness of the oil coating on the surface thereof. Surplus oil removed from the metering roller is able to return to the sump below.
Generally in prior art applicators the release oil is introduced into the supply trough at a single inlet usually at one end of the trough and distribution of the oil along the full length of the trough relies (a) on the oil reaching a level in the trough and (b) on the capillary capability of the wick. These processes tend to be relatively slow especially in view of the viscosity of the release oil and consequently points along the trough remote from the inlet may receive insufficient oil for stripping or may even suffer complete oil starvation particularly if the machine--and hence the trough--is tilted. Raising the amounts of oil and hence the oil level in the trough would aid distribution, but this is an undesirable solution because it increases the risk of oil spillage. The provision of additional oil inlets along the length of the trough would also aid distribution but this would increase cost and may not be possible if stringent space constraints have to be observed.